The invention relates to a multiband planar antenna intended for small-sized radio devices. The invention also relates to a radio device with an antenna according to the invention.
Models that operate in two or more systems using different frequency ranges, such as different GSM systems (Global System for Mobile telecommunications) have become increasingly common in mobile stations. The basic condition for the operation of a mobile station is that the radiation and receiving properties of its antenna are satisfactory on the frequency bands of all the systems in use. This is a demanding task when the antenna is located inside the covers of the device for comfort of use.
The internal antenna of a small-sized device often has a planar structure, because then the required properties are achieved most easily. The planar antenna includes a radiating plane and a ground plane parallel with it. In order to facilitate the matching, the radiating plane and the ground plane are generally connected to each other at a suitable point by a short-circuit conductor, whereby a structure of the PIFA (planar inverted F-antenna) type is created. The number of operating bands can be increased to two by dividing the radiating plane by means of a non-conductive slot into two branches of different lengths as viewed from the short-circuit point such that the resonance frequencies corresponding to the branches are in the range of the desired frequency bands. However, in that case the matching of the antenna can become a problem. Especially making the upper operating band of the antenna sufficiently wide is difficult when it is wanted to cover the bands used by two systems. One solution is to increase the number of antenna elements: An electromagnetically coupled, i.e. parasitic planar element is placed close to the main radiating plane. Its resonance frequency is arranged e.g. close to the upper resonance frequency of the two-band PIFA so that a uniform, relatively wide operating band is formed. Naturally, a separate third operating band can be formed for the antenna with the parasitic element. The use of a parasitic element has the drawback that even a small change in the mutual location of the element and the main radiating plane deteriorates the band properties of the antenna significantly. In addition, the parasitic element requires its own short-circuit arrangement.
On the other hand, the radiating plane itself can be shaped so that it also forms a third usable resonator together with the ground plane. FIG. 1 shows an example of such a solution. There is an internal multiband planar antenna with three separate operating bands, known from the application publication FI 20011043. The antenna 100 comprises a ground plane 110 and a radiating plane 120 with a rectangular outline. At the feeding point FP the radiating plane is galvanically coupled to the antenna feed conductor and at the short-circuit point SP to a short-circuit conductor that connects the radiating plane to the ground plane. The antenna is thus of the PIFA type. The feeding point FP and the short-circuit point SP are relatively close to each other on one long side of the radiating plane. On the radiating plane 120 there is a first slot 131 starting from its edge beside the feed point and ending at the opposite side of the plane, and a second slot 132 starting from the same edge beside the short-circuit point and ending at the central area of the plane. The feeding point and the short-circuit point are between these slots. As viewed from the short-circuit point SP, the slots 131 and 132 divide the plane into a first branch 121 and a second branch 122. The first branch is dimensioned so that together with the ground plane it forms a quarter-wave resonator and operates as a radiator on the lowest operating band of the antenna. The dimensioning is facilitated by an extension E1 directed towards the ground plane and additional bends E2 arranged in the first branch which extension and bends increase the physical and electrical length of the branch. The second branch 122 is dimensioned so that together with the ground plane it forms a quarter-wave resonator and operates as a radiator on the middle operating band of the antenna. The highest operating band of the antenna is based on the second slot 132, which together with the surrounding conductor plane and the ground plane forms a quarter-wave resonator and thus operates as a slot radiator.
The conductor patterns of the radiating plane 120 have been formed on an antenna circuit board 105, in a conductor layer on its upper surface. The antenna circuit board is naturally supported at a certain height from the ground plane 110.
The structure according to FIG. 1 has the drawback that the matching of the antenna on the lowest operating band leaves room for improvement. In addition, the structure does not allow to move the middle and the highest resonance frequency close to each other for forming a uniform and serviceable, wide operating band.
FIG. 2 shows another example of an internal multiband planar antenna known from the application publication Fl 20012045. The antenna 200 comprises a ground plane 210 and a radiating plane 220 with a rectangular outline. At the feed point FP the radiating plane is galvanically coupled to the antenna feeding conductor and at the short-circuit point SP to a short-circuit conductor that connects the radiating plane to the ground plane. The feed point FP and the short-circuit point SP are relatively close to each other on one long side of the radiating plane. In the radiating plane 220 there is a first slot 231 starting from its edge between the feed point and the short-circuit point and ending at the opposite side of the plane, and a second slot 232 starting from the same edge, from the other side of the feed point as viewed from the short-circuit point.
The antenna 200 has two operating bands and three resonances that are significant with regard to its use. The radiating plane 220 has a conductor branch 221 starting from the short-circuit point SP and going round the end of the second slot 232, which together with the ground plane forms a quarter-wave resonator and operates as a radiator on the lower operating band of the antenna. The second slot 232 is located and dimensioned so that together with the surrounding conductor plane and the ground plane it forms a quarter-wave resonator and operates as a radiator on the upper operating band of the antenna. The first slot 231 is also dimensioned so that together with the surrounding conductor plane and the ground plane it forms a quarter-wave resonator and operates as a radiator on the upper operating band of the antenna. The resonance frequencies of the two slot radiators are thus arranged relatively close to each other, but different so that the upper operating band becomes relatively wide. The frequency of the resonance based on the first slot 231 has also been arranged to a suitable point by means of a conductor plate E1, which is directed from the shorter side of the radiating plane 220 closest to the short-circuit point towards the ground plane.
In this example, the radiating plane is a metal sheet supported on a certain height from the ground plane with a dielectric frame 270.
In the structure according to FIG. 2, the upper operating band of the antenna is provided with two strong and separately tunable resonances. A very broad bandwidth is thereby obtained. However, this is achieved partly at the expense of the matching on the lower operating band, which is the drawback of that solution. In very small-sized devices, the lower band matching is already difficult because of the small size of the ground plane of the device.